This invention relates to an improvement in the process for the production of plastic-coated inorganic substances. More particularly, this invention relates to a process for the production of plastic-coated inorganic granular substances which comprises polymerizing in a gaseous phase a monomer carried on an inorganic granular substance.
Hitherto, plastic-coated substances have been produced by mixing a monomer, a catalyst and a core substance each in an aerosol form or by introducing a monomer into a suspension of particles of a core substance in a gaseous phase and polymerizing the monomer in the presence of a catalyst. In these prior art processes, however, polymerization of a monomer does not necessarily take place on the surface of the core particles and dispersion or local agglomeration of the resulting polymer takes place in the gaseous phase. Accordingly, these prior art processes suffer from the serious drawback that even or uniform coatings of the polymer on the core particles are attained only with extreme difficulty.
To overcome such serious drawbacks in prior art processes, the present inventors already proposed a wet process for producing inorganic substances coated with plastics wherein an inorganic granular material is suspended in a liquid containing a vinylic monomer and bisulfite ions and polymerization of the monomer is allowed to take place on the surface of the inorganic granules without the necessity of using any polymerization initiator. According to this process, the drawbacks in prior art processes are remarkably overcome and inorganic granular substances evenly coated with a plastic polymer can be obtained in a comparatively simple manner. As this wet process is carried out in an aqueous medium, however, additional after-treatments such as washing and drying are required for recovering the resulting product. Moreover, this wet process has the disadvantage that it cannot be applied to those inorganic substances which are dimensionally unstable or soluble in water. Thus, there is a need for modifying this wet process so that it may be applied to inorganic substances without using any liquid medium.